It has previously been proposed--see the referenced U.S. Pat. No. 4,053,188, JONNER, the disclosure of which is hereby incorporated by reference, to obtain a reference signal by sensing the speed of various wheels, and of first selecting the higher one of two wheel speeds and then changing the reference signal in accordance with time relationships stored, in the microprocessor. The time relationships, that is, the time constants of change, may be mathematically reprsented by curves. The curves representing the time constants may have different slopes which, then, change the reference signal in respectively different time rates. Change-over to the respective time constants is carried out, in accordance with this patent, based on wheel speed signals which are compared with the basic reference signal and with other criteria as follows:
(1) The mathematical curve will have a positive slope when a wheel speed signal is larger than the initial reference signal; PA0 (2a) The slope will be highly negative if both wheel signals are smaller than the initial reference value; and PA0 (2b) A small negative slope will be effective if braking pressure is maintained constant or is being decreased under control of an anti-brake lock system (ABS).
Additional time constants, for differently sloped curves may also be provided, and the selection of the curve or slope may additionally be influenced by other factors.
The foregoing brief explanation shows that the positive slope has priority; it is only necessary that the speed signal from one wheel is larger than the reference signal in order to render the positive slope effective.
The system to carry out this arrangement as described in U.S. Pat. No. 4,053,188 includes a memory for storing the reference value; first and second signal inputs for receiving respective first and second input signals whose values are dependent on the respective rotational speeds of two different wheels of the vehicle; a selector circuit, connected to the first and second signal inputs, for feeding the one of the first and second input signals with the greater value to the memory to cause the memory to store the greater value; a comparison circuit, connected to the first and second inputs and to the memory, for comparing the values of the first and second signals with the value of the reference signal stored in the memory and for emitting respective output signals whenever the value of either of the first and second signals exceeds or falls below the value of the reference signal stored in the memory; third and fourth signal inputs for receiving respective signals indicating that the control members associated with the wheels from which the first and second signals are derived have responded; and a control logic circuit, which is responsive to the output signals from the comparison circuit and to signals at the third and fourth signal inputs, for controlling the memory to cause the reference value stored in the memory to be selectively varied according to either a first negative time constant for rapidly decreasing the value in the memory whereby a very rapid matching of a higher reference value to the rotational speed of the wheel will be effected, or a second negative time constant for very slowly reducing the value in said memory whereby a very slow reduction in the reference value corresponding to a decrease in the speed of the vehicle on ground having a small coefficient of friction will be effected, or a third positive time constant for relatively rapidly increasing the value in the memory whereby a relatively rapid matching of the reference value with a greater rotational speed of a wheel will be provided. The control logic circuit causes the reference value stored in the memory to be varied according to the first time constant when the comparison circuit produces output signals indicating that the value of both of the first and second signals is less than said reference value stored in the memory; the second time constant when the comparison circuit produces output signals indicating that the value of both of the first and second signal is less than the reference value and signals are present at the third and fourth signal inputs indicating that both control members have responded; and the third time constant, when the comparison circuit produces output signals indicating that the value of at least one of the first and second signals is greater than the stored reference value.
The signals derived from two wheels are utilized to form the reference value for the slip measurement of these wheels. The wheels of one axle, of one side of the car and particularly the wheels which are diagonal to one another can be selected for this purpose.
Advantageously the above-mentioned switching devices of the control logic circuit which effects the switching in of the first time constant are effective independently of whether the vehicle brakes are being activated or not. This may also apply for the switching devices which make the second time constant effective.
Preferably, in addition to the three time constants mentioned above, the control logic circuit also can provide a fourth time constant having a positive sign which permits the reference value in the memory to increase only very slowly. This fourth time constant is made effective to control the memory by the logic circuit in both wheel speed values exceed the reference value in the memory at least by a certain value (+.lambda.). The second and fourth time constants may be of the same magnitude and may differ only as to their sign.
Finally, according to a further feature of the invention, the control logic circuit also provides a fifth time constant which has a positive sign and whose magnitude lies between the fourth and the third time constants. This fifth time constant is made effective by the logic circuit if either of the wheel speed values is greater than the reference value in the memory and neither of the wheel speed values deviates from this reference value by more than the predetermined value (+.lambda.).